Epoxy resins are known for their good corrosion protection properties, and therefore are frequently used as primers for metals. Similarly, the corrosion-inhibiting effect of phosphoric acid has been known for a long time. The combination of both classes of substance in one resin system is also not novel.
In the existing processes for preparing epoxy resin-phosphoric acid adducts a problem arises through the uncontrolled reaction of both components, resulting either in high molar mass reaction products or in unreacted phosphoric acid. Thus, for example, it is not possible to react an epoxy resin having a defined epoxide group content with phosphoric acid in an amount-of-substance ratio of 1 mol of phosphoric acid per 1 mol of epoxide groups deliberately in such a way, without leaving a considerable fraction of the phosphoric acid unreacted.
If the ratio of the amount of substance of phosphoric acid to the amount of substance of epoxide groups is reduced to below 1:1, there is less free phosphoric acid left at the end, but the uncontrolled increase in the degree of polymerization of the adduct becomes higher, a situation which can easily lead to gelling of the batch and can lead to products completely impossible to use.
The reason why the synthesis of the adduct is so difficult to control is that it depends heavily on the solvent used, on the nature of the epoxy resin, on the reaction temperature, on the addition time, and probably on certain other factors such as the reactor geometry.
Nor is the objective achieved by raising the amount-of-substance ratio of phosphoric acid to epoxide groups to more than 1:1. Although the increase to high molar masses in the formation of the adduct is stopped, the amount of free phosphoric acid which remains is larger. Residues of free phosphoric acid in the system are undesirable, since they impede the neutralization of the reaction product with amine (to obtain dilutability in water) and hence lead indirectly to poorer solubility of the product in water. Residues of free phosphoric acid are also a disruption in the cured coating film, particularly as a result of surface defects and floating.